Persistent Organic Pollutants

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Persistent organic pollutants

Persistent organic pollutants (POPs) are man-made organic compounds that persist in the natural environment for long periods of time. Because of their long-lasting presence in air, water, and soil , they accumulate in the bodies of fish, animals, and humans over time. Exposure to POPs can create serious health disorders throughout the tiers of the food web. In human beings, POPs can cause cancer , autoimmune deficiencies, kidney disorders, birth defects , and other reproductive problems.

Because these chemicals are derived from manufacturing industries, pesticide applications, waste disposal sites, spills, and combustion processes, POPs are a global problem. Many POPs are carried long distances through the atmosphere . They tend to move from warmer climates to colder ones, which is why even remote regions such as the Arctic contain significant levels of these contaminants. Because of the global creation and transmission of POPs, no country can protect itself against POPs without assistance. An international commitment is essential for eradication of this problem.

In the early 1990s the Organization for Economic Cooperation and Development (OECD), United Nations Environment Programme (UNEP), World Health Organization (WHO), and other groups began to assess the impacts of hundreds of chemicals, including POPs. In 1998 36 countries participated in the POPs Protocol, sponsored by the Convention on Long-range Transboundary Air Pollution . The purpose was to build an international effort toward controlling POPs in the environment.

In 2001, led by the UNEP, over 100 countries participated in the Stockholm Convention on Persistent Organic Pollutants. The proposal will become a legally binding agreement upon its ratification by 50 or more countries. It is considered by many to be a landmark human-health document of international proportions. Research will address the most effective ways to reduce or eliminate POP production, various import/export issues, disposal procedures, and the development of safe, effective, alternative chemical compounds.

The screening criteria for POP designation are: the potential for long-range atmospheric transport, persistence in the environment, bioaccumlation in the tissues of living organisms, and toxicity. Many chemicals volatilize, which increases the concentrations of these chemicals in the air. Longevity is measured by a chemical's half-life , or how much time passes before half of the original amount of a chemical discharge or emission breaks down naturally and dissipates from the environment. The minimum half-life for a POP in water is two months; for soil or sediment it is about six months. Several POPs have half-lives as long as 12 years. Animals accumulate POPs in their fatty tissue (bioaccumulation ); these POPs are then consumed and reconcentrated by higher-order animals in the food chain (biomagnification ). In some species , biomagnification can result in concentrations up to one million times greater than the background value of the POP. Most humans are exposed through consumption of food products (especially meat, fish, and dairy products) that contain small amounts of these chemicals.

The Stockholm Convention calls for the immediate ban on production and use of 12 POPs (known as the "dirty dozen"): aldrin, dieldrin, endrin, DDT, chlordane , heptachlor, mirex , toxaphene , hexachlorobenzene (HCB), polychlorinated biphenyls (PCBs), polychlorinated dioxins, and furans . The convention did acknowledge, however, certain health-related use exemptions until effective, environmentally friendly substitutes can be found. For example, DDT can still be applied to control malarial mosquitoes subject to WHO guidelines. Electrical transformers that contain PCBs can also be used until 2025, at which point any old transformers that are still active must be replaced with PCB-free equipment.

A brief description of the 12 banned POPs is shown below:

  • Aldrin was a commonly used pesticide for the control of termites, corn rootworm, grasshoppers, and other insects. It has been proven to cause serious health problems in birds, fish, and humans. Aldrin biodegrades in the natural environment to form dieldrin, another POP
  • Dieldrin was applied extensively to control insects, especially termites. Its half-life in soil is about five years. Dieldrin is especially toxic to birds and fish.
  • Endrin, another insecticide, is used to control insects and certain rodent populations. It can be metabolized in animals, however, which reduces the risk of bioaccumulation. It has a half-life of about 12 years.
  • DDT, also known as dichlorodiphenyl trichloroethane, was applied during World War II to protect soldiers against insect-borne diseases. In the 1960s and 1970s its use on crops resulted in dramatic decreases in bird populations, including the bald eagle . DDT is still being manufactured today and is a chemical intermediate in the manufacture of dicofol. Regions that want to continue using DDT for public-health purposes include Africa, China, and India (mosquitoes, however, are becoming resistant to DDT in these areas).
  • Chlordane is a broad-spectrum insecticide used in termite control. Its half-life is about one year. Chlordane is easily transported through the air and suspected of causing immune system problems in humans.
  • Heptachlor was applied to control fire ants , termites, and mosquitoes and has been used in closed industrial electrical junction boxes. Its production has been eliminated in most countries.
  • Mirex was used as an insecticide to control fire ants and termites. It is also a fire-retardant component in rubber , plastics , and electrical goods. It is a very stable POP with a half-life of up to 10 years.
  • Toxaphene is an insecticide that was widely used in the United States in the 1970s to protect cereal grains, cotton crops, and vegetables. Toxaphene has a half-life of 12 years. Aquatic life is especially vulnerable to toxaphene toxicity.
  • Hexachlorobenzene, also called HCB, was introduced in the 1940s to treat seeds and kill fungi that damaged crops. It can also be produced as a by-product of chemical manufacturing. It is suspected of causing reproductive problems in humans.
  • Polychlorinated biphenyls, commonly referred to as PCBs, were used extensively in the electrical industry as a heat exchange fluid in transformers and capacitors. They were also used as an additive in plastics and paints. PCBs are no longer produced but are still in use in many existing electrical systems. Thirteen varieties of PCBs create dioxin-like toxicity. Studies have shown PCBs are responsible for immune system suppression, developmental neurotoxicity, and reproductive problems.
  • Polychlorinated dibenzo-p-dioxins, usually shortened to the term "dioxins," are chemical by-products of incomplete combustion or chemical manufacturing. Common sources of dioxins are municipal and medical waste incinerators, backyard burning of trash, and past emissions from elemental chlorine bleach pulp and paper manufacturing. A diox in's half-life is typically 1012 years. Related health problems can include birth defects, reproductive problems, immune and enzyme disorders, and increased cancer risk.
  • Polychlorinated dibenzofurans, usually shortened to "furans" or "PCDFs," are structurally similar to dioxins and by-products of the same processes as for dioxins. The health impacts of dibenzofuran toxicity are considered to be similar to those of dioxins. Over 135 types of PCDFs are known to exist.

The UNEP and other world organizations are continuing to look for safer and more economically viable alternatives to POPs. Manufacturing facilities are being upgraded with cleaner technologies that will reduce or eliminate emissions. Focus is also being placed on regulating the international trade of hazardous substances. Disposal is also an issue; poorer countries don't have the money or proper technological resources to dispose of the growing accumulations of obsolete toxic chemicals. The UNEP is working creatively with countries to secure financing to introduce alternative products, technology, methods enforcement, and critical infrastructure. Research sponsored by UNEP studies the chemical characteristics of current POPs so that chemical companies will be less likely to create new POPs.

The Convention on Long-range Transboundary Air Pollution has been extended by eight protocols, including the 1998 protocol on POPs. The third meeting of an adhoc group of experts on POPs was held in 2002. These scientists continue to research other chemicals that may qualify in the future as POPs.

Current technical review activities focus on the following chemicals:

  • Ugilec, which was used in capacitors, transformers, and as a hydraulic fluid in underground mining. It was originally thought to be a safe replacement for PCBs.
  • Hexachlorobutadiene (HCBD) was used to recover chlorine-bearing gas from chlorine plants. It is very toxic to aquatic life. HCBD is on national priority lists for some countries, such as Canada.
  • Pentabromodiphenyl ether (PentaBDE) is sold as a brominated flame retardant. Its use has doubled in the last 1015 years. This compound volatilizes easily and enters the air long after the disposal of treated products.
  • Pentachlorobenzene (PeCB) was used as a fungicide , flame retardant, and as a component in dielectric fluids. Although it is no longer being produced, there is still an abundance of PeCB in the environment. It is probably released through landfill leaching/degassing or incineration.
  • Polychlorinated naphthalenes (PCNs) are structurally similar to PCBs. In the 1980s PCNs were a component of products called Halowax, Nibren Wax, and Seekay Wax. Uses included wood preservation, electroplating, dye carriers, and wire insulation. Major points of release are old electrical equipment or incineration.
  • Short-chain chlorinated paraffins (SCCP) are still being used today in the rubber, leather, and metal-working industries, especially in China. No suitable substitutes have been developed yet.
  • Polychlorinated terphenyls (PCTs) are very similar in structure and chemical behavior to PCBs. Over 8,000 different varieties of PCTs theoretically exist. PCTs have not been commercially produced since the 1980s, but are still part of aging electrical capacitors and transformers.

[Mark J. Crawford ]



Draft Summaries for Discussion by the Expert Group on POPs, Protocol on POPs. Summary documents. United Nations Economic Commission for Europe, Environmental and Human Settlements Division, June 2002.

The Foundation for Global Action on Persistent Organic Pollutants: A United States Perspective. External review draft. U.S. Environmental Protection Agency, November 2001.

New Protocol on Persistent Organic Pollutants Negotiated. News release. U.S. Environmental Protection Agency, Office of Pesticide Programs, 1998.

Persistent Organic Pollutants: A Global Issue, a Global Response. Brochure. Document EPA 160-F-02-001. U.S. Environmental Protection Agency, Office of Internal Affairs, 2002.

Stockholm Convention on POPs. Pamphlet. United Nations Environment Programme, June 2001


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